Then they should've had some discussion about it at some point, something like "These Borg creatures were once considered out greatest threat. Nowadays we know that they're just one of many."

They weren't, so why would they say that they were? Of all the threats Q alluded to, the Borg WERE the only actual threat to them because the Borg were the only ones who knew about their existence and had any reason to come after them.